Against the background of increasingly scarcer fossil fuels the conversion of biomass into an easy to distribute gas offers an economical alternative. If the primary synthesis gas that results is then further converted into hydrogen, and distributed to the end customer, decentralised power generation by means of fuel cells is even possible. The production and use of hydrogen requires a very pure and low-nitrogen synthesis gas. These requirements also apply for other chemical syntheses.
For the thermochemical production of synthesis gas from biomass essentially three methods are known.
For the low power range, fixed bed gasifiers in a number of variants predominate. Fixed-bed gasifiers are reliant on a consistently high quality of the biomass and are not suitable for the production of high-quality synthesis gas which is suitable for further processing into hydrogen.
The entrained-flow gasifier is particularly well-suited to high powers of 1 GW upwards, since the reactor size of the entrained-flow gasifier is relatively small. For small plants the entrained-flow gasifier is uneconomic because of the high-cost of the equipment. The entrained-flow gasifier requires extensively dry biomass or primary products, because at high temperatures the entrained-flow gasifier works with pure oxygen. The ashes sinter like glass and are unsuitable for use as a mineral fertilizer. This is a problem given that fertilizers are becoming more expensive and more scarce.
The fluidized bed reactor comes into its own in the medium industrial power range of 1 MW to 1 GW. The ash from fluidized-bed reactors can be used as a mineral fertilizer in agriculture. In methods with fluidized bed reactors a distinction can be made between autothermal and allothermal gasification.
In autothermal gasification part of the biomass is burned in the fluidized bed reactor to cover the expiring endothermic reactions. The autothermal gasification is air-operated. Pure oxygen would lead to localised overheating in the fluidized bed. Methods which use air as the fluidization gas are therefore not so easy to convert to oxygen. The use of air leads to dilution of the synthesis gas with nitrogen and CO2, making its use for power generation and further processing into products of hydrogen, methane, methanol or liquid fuels more difficult.
With allothermal gasification the necessary heat is introduced through heat transfer. This can take place, for example, by means of heating rods in the fluidized bed, as described in DE 199 26 202 C1. Heat transfer media circulating between a burner and a synthesis gas reactor are also known. As a heat transfer medium sand is usually used, which is heated in a second reactor by combustion of part of the biomass. Such a gasifier with a thermal power of 8 MW can be found in Güssing, Austria. This plant was presented at the 1st International Ukranian Conference on BIOMASS FOR ENERGY; Sep. 23-27, 2002 Kyiv, Ukraine by M. Bolhar-Nordenkampf et al. under the title: “Scale-up of a 100 KWth pilot FICFB to 8 MWth FICFB-gasifier demonstration plant in Güssing (Austria)”. Steam is used as the fluidizing gas for the synthesis gas reactor. The provision of steam calls for an additional expenditure of energy, in many cases reduces the efficiency and increases investment costs.
As a rule the biomass is fed into the synthesis gas reactor directly, leading to a very high tar content, because coarse parts of the biomass after a few seconds can reach the upper part of the fluidized bed and the emitted gases containing tar that are formed reach the synthesis gas directly. This makes in any case an expensive method for removal of the tar necessary. The recovery of sensible heat is only possible to a limited extent, because at below 350° C. tar deposits on the walls of the apparatus. The problem of tar is currently the greatest drawback in gasification of biomass.
From DE 601 20 957 T2 a method is known in which biomass is first decomposed in a pyrolysis reactor into pyrolysis coke and pyrolysis gas in order then to pass these two to a reactor for generation of synthesis gas. The method is tailored to the use of air and therefore has a high quantity of nitrogen in the synthesis gas. Due to the design and operation of the pyrolysis reactors quite coarse coke reaches the synthesis gas reactor. This coarse coke is not yet fully degassed and causes a high tar content in the synthesis gas. The tar and nitrogen content however, following simple washing, meets the requirements for combustion engines, but meets not the requirements for production and use of hydrogen and the conducting of other chemical syntheses.
It is an object of the invention to largely avoid the disadvantages described and through primary measures to provide a low-tar synthesis gas with a high yield.